Which issue does this PR close?

• N/A

Rationale for this change

DataFusion publishes Apache Spark–compatible functions and expression planners in the datafusion-spark crate, but datafusion-java had no way to use them. This adds an opt-in so Java callers can build a SessionContext whose SQL surface matches Spark semantics (e.g. crc32), without affecting the default context.

What changes are included in this PR?

• Java: a new SessionContextBuilder.withSparkFunctions() setter, carried through the existing SessionOptions protobuf path. It composes with all other builder options; the no-arg new SessionContext() path is unchanged (Spark functions are strictly opt-in).
• Proto: a new optional bool spark_functions field on SessionOptions.
• Native: a build_session_context helper that, when the flag is set, builds the context through SessionStateBuilder::with_default_features().with_spark_features() (so Spark implementations override built-ins of the same name). Object-store registration and the memory tracker are preserved.
• Cargo: the datafusion-spark (v53.1.0, matching the pinned DataFusion) dependency behind a new optional spark feature, included in the default feature set — mirroring the existing object-store feature gating. Builds that strip the feature surface a clear error from the JVM if the flag is requested.
• Docs: a "Spark-compatible functions" section in the SessionContext user guide.

Are these changes tested?

Yes.

• Java: proto round-trip tests for the flag, plus an integration test asserting crc32('Spark') returns 1557323817 with the flag and that the same SQL fails without it (proving the flag is load-bearing).
• Rust: unit tests gated on the spark feature confirming crc32 is registered when the flag is set and absent otherwise.
• Full suites pass: Rust cargo test --features spark, JVM ./mvnw -pl core test (334 tests), and cargo clippy --features spark -- -D warnings clean. Verified the native crate also compiles with --no-default-features.

Are there any user-facing changes?

Yes — a new public SessionContextBuilder.withSparkFunctions() method and accompanying user-guide documentation. No breaking changes to existing APIs.